arXiv:1604.07624v1 [astro-ph.GA] 26 Apr 2016 uy2021 July 9 ic etclrglxe oeie oss nyodthick old only possess ( sometimes thin discs starforming galaxies the Lenticular to compared disc. stars poor metal more and MNRAS sarsl fhg est n eoiydseso of dispersion velocity and redshifts density high high ⋆ at of rapidly result formed a discs Thick as (i) posed. ( quenchi through spirals galaxies of lenticular the of scenario mation of scale-height the with nent hndssadbecome and discs de thin (see redshifts high in at discs tails thick formed they where nario ( cases all not they if plane, disc most 2002 main in the found above dis are distances edge-on large in light at of galaxies excess exponential the by Manifested Silchenko, K. Olga
nsai .Kasparova, V. Anastasia Discs Galactic Thick of Diversity The lmnso prladlniua aais( galaxies lenticular and structural widespread spiral and of important elements discs, thick for of the is ofmation problem unsolved complexity such One and simulations. numerical resolution increasing i astronomical and the in strumentation progress great fully the not despite remain evolution understood galaxy of aspects important Many INTRODUCTION 1 c 3 2 1 5 4 sa etnIsiueo hl,Mso rnh Universite Branch, Moscow Chile, of Institute Newton Isaac Harvard-Smithso S Observatory, Lomonosov Astrophysical M.V. Smithsonian Moscow Institute, Astronomical Sternberg eateto hsc,Mso ..LmnsvSaeUnivers State Lomonosov M.V. Moscow Sci Physics, of of Academy Department Russian Observatory, Astrophysical Special otc e-mail: Contact 06TeAuthors The 2016 eea hc icfrainseaishv enpro- been have scenarios formation disc thick Several .TeMlyWycnan hc tla iccompo- disc stellar thick a contains Way Milky The ). cemde l 2015 al. et McDermid i’hnoe l 2012 al. et Sil’chenko 000 asne l 1980 al. et Larson , 1 – 6 [email protected] 21)Pern uy22 oplduigMRSL MNRAS using Compiled 2021 July 9 Preprint (2016) normal 1 ). acquired never have then and ) en ossetwt sce- a with consistent being ) , ∼ 3 prl ncnrs otefor- the to contrast in spirals h ig lse,adNC52 nasas ru.W e intermedia see We edg group group. sparse three dense a in a in th in 5422 discs NGC Using 4111 thick and NGC cluster, populations. of environments: Virgo stellar the different spectra ga their in deep disc located collected about observ galaxies edge-on we to information BTA Due all of telescope debate. lack nearly m of a in matter is a detected there remain are still scenarios discs formation stellar thick Although ABSTRACT ea ih([Fe/H] rich metal nteohrhn,NC52 osnthroryugsas t dis its stars, its and young component, harbour second a not for does evidence without 5422 NGC Gyr), (10 hand, old other the On u eut ugs h iest ftikds omto scenarios formation disc words: thick Key of diversity the suggest results Our ugssa1 a suggests lxyV Moiseev V. Alexey p hthror older harbours that kpc 1 acno Bernstein & Dalcanton 1 ⋆ vnY.Katkov, Yu. Ivan , usen1979 Burstein . 5 − aais vlto;glxe:srcue aais stel galaxies: structure; galaxies: evolution; galaxies: y ogfraineohipyn t omto thg redshift. high at formation its implying epoch formation long Gyr 2 − ∼ inCne o srpyis 0Gre t S9 Cambridge MS09, St. Garden 60 Astrophysics, for Center nian ne,NznjAky,396,Russia 369167, Arkhyz, Nizhnij ences, ng 0 n- si r,1,Mso,199,Russia 119992, Moscow, 13, pr., tskij ). c . - - aeUiest,Uiesttkjp. 3 ocw 119992, Moscow, 13, pr., Universitetskij University, tate 2 . . . 4 t,1 eisi oy ocw usa 119991 Russia, Moscow, Gory, Leninskie 1, ity, , 1 0 tv ailaegain bv h rnia icplane disc principal the above gradient ne ( age a example, radial for patterns, ative population stellar vertical and old produce should nario ue l 2011 al. et Qu 2013 al. Roˇskar et 2014 Combes ( 2009 Universe al. early et Bournaud the in gas i eua hnds aiga euto ailmigration radial of result a ( as stars flaring disc formof be thin can secular discs via Thick (ii) gradients. metallicity notable eovdselrknmtc n trfrainhistories. spatially derived formation and star this galaxies and disc kinematics edge-on stellar resolved of sample a for surface low typical discs. of thick because of repr brightnesses challenge it However, significant studies. a disc sents obse the thick only for choose the technique int to remains vational resolved spectroscopy us be light help cannot integrated which will stars, galaxies discs external thick In and scenario. thin in ulations ( ers ygsaceinfo lmns(see filaments from accretion gas by yaial etdb aelt yy ( flybys satellite by heated dynamically . n vaolvB Borisov B. Svyatoslav and e)selrppltosi G 11adNC4710. NGC and 4111 NGC in populations stellar dex) 0 ice ta.2015 al. et Minchev 1 oeto ta.2006 al. et Robertson ecridotde ogsi pcrsoi observations spectroscopic long-slit deep out carried We pop- stellar and kinematics internal of studies Detailed letter grV Chilingarian, V. Igor Sch epeettefis eut ntreS- galax- S0-a three on results first the present we nih&Bne 2009 Binney & ¨ onrich n/rmnrmergers. minor and/or ) n eeecsteen rminor or therein) references and .Sm cnro rdc pcfi radial specific predict scenarios Some ). .(i)Piodal hndsscnget can discs thin Primordially (iii) ). ,adti ic omdconsequently formed discs thin and ), ; α i’hnoe l 2011 al. et Sil’chenko ehne hc ic without discs thick -enhanced lere lere 2006 Elmegreen & Elmegreen ; α hapn ta.1997 al. et Chiappini 2 oba ta.2011 al. et Loebman eeetabundance -element . , a content lar toa difficulties, ational 1 1 un ta.1993 al. et Quinn eae(4 age te A , 5 T . E stikand thick is c G 70in 4710 NGC , -nS- disc S0-a e-on tl l v3.0 file style X usa 6- Russian e ais their laxies, Russia .Ti sce- This ). A018USA 02138 MA , wet − Gyr) 5 merg- ed In g- e- r- o ; ; ; ; 2 A.Kasparova et al.
Table 1. Long-slit spectroscopy of the sample galaxies. We reduced our spectroscopic observations with our own idl-based reduction pipeline. We estimated the night
NGC Date z-offset P A Sp. range Texp Seeing sky background from outer slit regions not covered by our arcsec/pc deg A˚ sec arcsec target galaxies and an optimized sky subtraction technique
4111 21/05/09 0/0 150 4825-5500 8400 1.3 that takes into account spectral resolution variations along 4111 24/04/15 5/364 150 3600-7070 5600 1.0 the slit (Katkov & Chilingarian 2011; Katkov et al. 2014). 4710 24/04/15 0/0 27.5 3600-7070 3600 1.2 4710 24/04/15 7/560 27.5 3600-7070 7200 1.1 All three galaxies were observed with the Infrared Array 5422 24/04/12 0/0 151.4 3600-7070 3600 2.5 Camera (IRAC) at Spitzer Space Telescope in the imaging 5422 25/04/15 7/1049 151.4 3600-7070 8400 1.5 mode at wavelengths 3.6 µm and 4.5 µm. We fetched fully reduced 3.6 µm images from the Spitzer Heritage Archive1. ies which, as we show, prove the diversity of the thick disc formation scenarios.
2 THE SAMPLE AND THE DATA 2.1 The Sample We chose three edge-on galaxies in different environments: NGC 4111, NGC 4710 and NGC 5422 morphologically clas- 2.3 Data Analysis sified as S0-a by Hyperleda (Makarov et al. 2014). Van der Kruit & Searle (1981) have shown that in case an • NGC 4111 (MJ = −22.40 mag) is a member of the isothermal disc in the equilibrium state the vertical disc den- 2 Ursa Major galaxy group that is known to contain a com- sity profiles are described by the law I = I0 sech (z/z0), mon extended HI envelope (Wolfinger et al. 2013). The dis- where I0 is the central intensity and z0 is the disc scale- tances from NGC 4111 to the nearest neighbours are about height. We fitted vertical profiles obtained by averaging 30 − 40 kpc (Pak et al. 2014; Karachentsev et al. 2013). We Spitzer IRAC images along the radius using models includ- adopt the distance 15 Mpc that corresponds to the spatial ing one and two components. We set central positions of both −1 scale 72.7 pc arcsec (Tonry et al. 2001). components to be the same in the case of two-component • NGC 4710 (MJ = −22.56 mag) is located in the Virgo fitting but left the position itself a free parameter. Our re- cluster outskirts (d = 16.5 Mpc by Mei et al. (2007), spatial sults for NGC 4111 and NGC 5422 quantitatively agree with −1 scale 80.0 pc arcsec ). Its projected distance to M 87 is those presented in the S4G survey (Salo et al. 2015), how- about 5.4 deg or 1.6 Mpc (Koopmann et al. 2001). A dusty ever the results for NGC 4710 decomposition were not pre- disc is observed in the central 2 kpc region dominated by sented there. an X-shaped structure, that is traditionally explained as an To derive internal kinematics and stellar population edge-on bar (Bureau & Freeman 1999). properties (mean ages and metallicities [Fe/H]) of thick and • − NGC 5422 (MJ = 22.81 mag) is a member of the thin discs we first binned our long slit spectra in the spa- sparse NGC 5485 galaxy group dominated by lenticular tial direction with the adaptive binning algorithm in or- galaxies. It is the most luminous and the most distant ob- der to reach the minimal signal-to-noise ratio S/N = 30 ject in our sample (d = 30.9 Mpc by Theureau et al. (2007), per bin per spectral pixel in the middle of the wavelength −1 spatial scale 150 pc arcsec ). It possesses a large gaseous range. Then, in every bin we applied the nbursts full spec- ∼ disc tilted by some 5 deg with respect to the stellar disc. tral fitting technique (Chilingarian et al. 2007a,b) with a grid of high resolution stellar pegase.hr (Le Borgne et al. 2004) simple stellar population (SSP) models based on the 2.2 New Observations and Archival Data ELODIE3.1 empirical stellar library (Prugniel et al. 2007). We obtained deep spectroscopic observations of all The nbursts technique implements a pixel space χ2 min- three galaxies with the universal spectrographs SCOR- imization algorithm where observed spectrum is approxi- PIO (Afanasiev & Moiseev 2005) and SCORPIO-2 mated by a stellar population model broadened with para- (Afanasiev & Moiseev 2011) at the Russian 6-m BTA metric line-of-sight velocity distribution (LOSVD) and mul- telescope using the 1 arcsec wide 6 arcmin long slit. For tiplied by polynomial continuum (10th degree in our case) to every galaxy we observed: (i) a major axis in order to take into account dust attenuation and/or possible flux cal- get the information on a thin disc for NGC 4111 and ibration imperfections in both observations and models. We NGC 4710, and the mid-plane of the thick disc in NGC 5422 used Gaussian parametrization of the stellar LOSVD, except and (ii) a region parallel to the major axis offset by our higher resolution mid-plane spectra of NGC 4111 where 5 − 7 arcsec that corresponds to 0.36 − 1 kpc above the we exploited the standard Gaussian-Hermite parametriza- mid-plane in order to probe thick discs. We observed (see tion (van der Marel & Franx 1993). Table 1) the mid-plane of NGC 4111 with the SCORPIO The [Mg/Fe] abundance ratio which allows one to esti- in the wavelength range 4800 − 5600 A˚ with the spectral mate the duration of the star formation epoch is fixed to the resolution ∼ 2.2 ˚A =55 kms−1. The remaining 5 datasets solar value in the pegase.hr model grid. Therefore, we mea- were obtained using SCORPIO-2 at slightly lower spectral sured the Mgb, Fe5270/5335 Lick indices (Worthey et al. resolution (∼ 3.8 A=95kms˚ −1 at 5100 A)˚ at the broader 1994) and derived [Mg/Fe] ratios in several radial bins along wavelength range (3600 − 7070 A).˚ the slit using α-variable models from Thomas et al. (2003).
MNRAS 000, 1–6 (2016) The Diversity of Thick Discs 3
30 NGC4111 NGC4710 NGC5422 30 20 20 10 10 0 0 -10 -10 -20 1 kpc 1 kpc 1 kpc -20 -30 -30 z-offset, arcsec z-offset, 15 15 10 10 7 7 5 5 4 4
Age, Gyr Age, 3 3 2 mid-plane 2 thick disc 1 1 0. 2 0.2
0.0 0.0
-0.2 -0.2
-0.4 -0.4 Metallicity,dex -0.6 -0.6 200 200
100 100 N S SW NE N S 0 0
-100 -100 -200
Meanvelocity,km/s -200 200 200
150 150
100 100
50 50 Dispersion,km/s 0 0 -80 -60 -40 -20 0 20 40 60 80 -80 -60 -40 -20 0 20 40 60 80 -80 -60 -40 -20 0 20 40 60 80 Radial distances, arcsec Radial distances, arcsec Radial distances, arcsec
Figure 1. Stellar populations and internal kinematics of three edge-on disc galaxies. Panels (top to bottom): the slit positions overplotted on SDSS color images, SSP equivalent ages, metallicities, radial velocities and line-of-sight velocity dispersions. Black and green symbols denote mid-planes and thick discs.
3 RESULTS AND DISCUSSION asymmetry of the thick disc stellar population profiles in the outer regions is not statistically significant. With the excep- 3.1 NGC 4111 tion of the central region where our mid-plane data probes In Fig. 1 (left plots) we show the SSP-modelling re- the bulge and a kinematically decoupled co-rotating inner sults of NGC 4111 stellar population and kinematic data. disc (within 10 arcsec), we see a completely flat age profile. Comer´on et al. (2014) have estimated the thin and thick disc Metallicity has a slight negative gradient and decreases from 9 9 − − masses to be 15 ± 3 · 10 M⊙ and 5 ± 1 · 10 M⊙ respectively, [Fe/H] = 0.2 dex at r = 15 arcsec to 0.4 dex in the outer and Comer´on et al. (2012) have demonstrated that the thick regions of both discs. disc of NGC 4111 dominates the luminosity at z > 25.4 arc- sec (∼ 1.8 kpc) in the inner region. However, according to their models, already at r = 40 arcsec the thick disc con- 3.2 NGC 4710 tribution represents about 80 per cent of the total lumi- Our data extends to 85 arcsec (6.8 kpc) and we can iden- nosity fraction in the mid-plane and reaches 90 per cent at tify at least three distinct regions in NGC 4710 by radius: z = 5 arcsec were we placed the slit. Our data extend out (i) an inner dusty region within 30 arcsec that corresponds to 6.2 kpc from the centre that corresponds to 10.0 and 2.7 to the outer edge of a clearly seen X-shaped structure, thin and thick disc scale-lengths respectively. Nevertheless, (ii) a bright thin blue disc extending to 60 arcsec, and we do not see significant differences of stellar populations (iii) weaker outer regions. Mid-plane age and metallicity in the outer regions of the NGC 4111 discs. The [Mg/Fe] profiles confirm the stepped structure (black points on the values for both disc components are about +0.15 dex and centre panels of Fig. 1). A slight asymmetry within the cen- consistent within 0.03 dex (Fig. 2, top panel). Stellar ages of tral region corresponds to the bright spot distinctly seen on ∼ the two layers are almost identical 5 Gyr. Moreover, the optical images of NGC 4710 south-west of its centre. It is stellar velocity dispersion profiles are similar too. The slight probably a giant star formation region that shines through unevenly distributed dust. Outside the dusty region, be- tween 30 and 60 arsec, we see a young (∼ 2.5 Gyr) metal- 1 http://sha.ipac.caltech.edu/applications/Spitzer/SHA/ rich (∼ +0.1 dex) component without significant radial stel-
MNRAS 000, 1–6 (2016) 4 A.Kasparova et al.
0.25 N4710 0.2 N4111 1.5
0.15 1 0.1 [Mg/Fe] 0.05 0.5 mid-plane thick disc 0 scale−height, Vert. kpc 0 0 20 40 60 80 100 Radial distances, arcsec 0.3 N4710 Figure 3. Scale-height radial variations of the NGC 4710 thin 0.2 and thick discs. The designations are as in Fig. 2. [Mg/Fe] 0.1
0 3.3 NGC 5422 0.4 NGC 5422 has a single disc component as evident from its 0.3 vertical density profile analysis. The two-dimensional de- 0.2 composition by the S4G project reveals a bulge and one 0.1 thick disc with the scale-length and scale-heights 24.3 and [Mg/Fe] 0 6.7 arcsec (Salo et al. 2015). The disc warp semi-amplitude −0.1 N5422 reaches 110 pc at r = 3 kpc. NGC 5422 also possesses a large −0.2 scale non-starforming gaseous disc slightly inclined at 5 deg 0 20 40 60 80 100 Radial distances, arcsec to the main plane manifested by weak emission lines. For comparison with the mid-plane of NGC 5422 we Figure 2. The [Mg/Fe] values averaged in radial bins. Black and placed a slit at 7 arsec (∼ 1050 pc) above it. Our profiles green dots are for thin and thick discs, respectively. Dotted lines of kinematics and stellar population parameters extend to denote the three regions of NGC 4710, shaded gray is a bar dom- 70 arcsec (10.5 kpc). We expect that outside r = 20 arsec the
MNRAS 000, 1–6 (2016) The Diversity of Thick Discs 5 and, consequently, radial distances than in previous studies trum of their formation scenarios. Further clarifications on (Yoachim & Dalcanton 2008; Comer´on et al. 2015), there- preferred scenarios will be obtained when our sample of deep fore the flat radial age profiles which we observe can rule spectroscopic observations extends. out this formation model. On the other hand, this situation may occur if the radial migration was strong enough to flat- ten radial stellar population gradients in both thin and thick ACKNOWLEDGMENTS discs. The rapid turbulent thick disc formation by We are grateful to Anatoly Zasov for productive discus- Bournaud et al. (2009) looks plausible only for NGC 5422. sions and we thank the anonymous referee for useful com- This galaxy has a moderately α-enhanced very old stellar ments. The Russian 6m telescope is exploited under the fi- population in its disc that correspond to the duration of the nancial support by the Russian Federation Ministry of Ed- star formation epoch of 1.5 − 2 Gyr (Thomas et al. 2005), if ucation and Science (agreement No14.619.21.0004, project it had started to form at z ≈ 3 and was finished by z ≈ 1.5. ID RFMEFI61914X0004). Our deep spectroscopic observa- At the same time, we cannot clearly assess the impor- tions of thick discs are supported by the Russian Science tance of other thick disc formation mechanisms, such as mi- Foundation project 14-22-00041. The archival data analy- nor mergers or accretion events, for all three galaxies. For sis is supported by the grants MD-7355.2015.2 and RFBR example, very moderate negative or flat metallicity gradi- 15-32-21062. 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This paper has been typeset from a TEX/LATEX file prepared by the author.
MNRAS 000, 1–6 (2016)